In general, during manufacturing a liquid crystal display device, a polysilicon thin film is used to form an active layer of a thin film transistor in an array substrate. However, since a typical base substrate has a low heat resistance, if the polysilicon thin film is formed at a high temperature directly, the base substrate may be deformed. Therefore, a low temperature polysilicon thin film is used generally in forming the active layer of the thin film transistor.
An excimer laser annealing (ELA) method is one of the most popular methods for forming a low temperature polysilicon thin film for mass production, in which an excimer laser with a high energy is used to irradiate the amorphous silicon thin film, and the amorphous silicon is melted by absorbing the energy of the excimer laser and then is crystallized to form a polysilicon thin film after cooling, which is performed entirely at a temperature from 500° C. to 600° C. The ELA method is performed by scanning the amorphous silicon layer with a pulse laser of the excimer laser generator to form an irradiated region, and further forming a plurality of irradiated regions which overlap each other by moving the pulse laser forward at a certain interval in scanning. Since the overlapped portion has a higher temperature than that of the non-overlapped portion, a heterogeneous nucleation may occur at a boundary between the overlapped portion and the non-overlapped portion. The overlapped portion and the non-overlapped portion may generate a lateral temperature gradient, so the crystal core will grow in a direction in which the temperature is increased, that is, in a direction from the non-overlapped portion to the overlapped portion, and is crystallized to form a low temperature polysilicon thin film finally.
For example, as shown in FIG. 1, a buffer layer 106 is formed on a substrate 101, and an amorphous silicon layer is formed on the buffer layer 106. The amorphous silicon layer is crystallized to form a polysilicon layer 105 after performing an ELA process. However, the inventors have found that, since the ELA process is complicated, the amorphous silicon layer can not be crystallized totally to form a polysilicon layer, so that the formed polysilicon layer 105 has an uneven surface with a plurality of projection portions 104 formed thereon. When the thin film transistor is applied a voltage, the projection portions on the surface of the polysilicon thin film may result in a point discharge phenomenon, so the leakage current is increased. Furthermore, since the polysilicon thin film has an uneven surface and thus has large resistance, the mobility and the threshold voltage of the polysilicon thin film are not uniform, and the product quality is adversely affected.